Launch Xcomppmgr and tint2 with openbox

Other (better) ways to make Xcompmgr run at startup are discussed in the Openbox article.
Fullscreen/Overlay

To force tint2 to stay on top of the application (overlay), you need to set the panel_layer option appropriately. This can be helpful when you switch from a fullscreen window to a normal application using Alt-Tab. There is a discussion on this at Crunchbang Forum